1. Field of the Invention
The invention relates generally to the field of obtaining nuclear magnetic resonance (NMR) measurements from fluids. More specifically, the invention relates to the use of micro-NMR devices to obtain downhole NMR measurements.
2. Background Art
Nuclear magnetic resonance (NMR) can be used to determine various characteristics of subsurface formations and/or samples. Conventional NMR logging tools can be used downhole to obtain these characteristics, which then can be used to assist in the determination of, for example, the presence, absence, and/or location of hydrocarbons in a given formation or sample. Conventional NMR logging, well known in the art, generally involves deploying in a wellbore an NMR instrument, which uses a DC magnetic field to polarize Hydrogen nuclei (essentially protons) and an antenna to generate RF pulses and detect various RF signals from nuclei in a formation or sample. Certain exemplary NMR techniques are described in U.S. Pat. No. 6,232,778 assigned to Schlumberger Technology Corp., the entire disclosure of which is hereby incorporated by reference.
NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field B0 to the formation, such as through the use of permanent magnets. This field causes the proton spins to align in a direction parallel to the applied field (this step, which is sometimes referred to as the creation of longitudinal magnetization, results in the nuclei being “polarized”). Polarization does not occur immediately, but instead grows exponentially in accordance with a time constant T1, and may take as long as several seconds to occur. After sufficient time, a thermal equilibrium polarization parallel to B0 is established.
Next, a series of radio frequency (RF) pulses are produced so that an oscillating magnetic field, B1, is applied. The first RF pulse (referred to as the 90-degree or tipping pulse) rotates the magnetization from B0 direction substantially into the transverse plane (i.e., transverse magnetization). Additional RF pulses (often referred to as 180-degree or refocusing pulses) are applied to create a series of spin echoes. The frequency of the RF pulses is chosen to excite specific nuclear spins of a particular region of the sample that is being investigated.
Two time constants are associated with the relaxation processes of the longitudinal and transverse magnetization: T1 and T2. The spin-lattice relaxation time (T1) is the time constant for longitudinal magnetization to return to its thermal equilibrium value in the static magnetic field. The spin-spin relaxation time (T2) is the time constant for the transverse magnetization to return to its thermal equilibrium value which is zero. The spin echo intensity versus time, collected by conventional NMR logging tools, is normally decomposed and then displayed in relaxation or T2 space. Various conventional methods exist for decomposing spin echoes to be displayed in T2 space.
Recently, micro-NMR devices have been developed that utilize many of the same principles of conventional NMR logging tools. These micro-NMR devices can be used in surface and downhole applications, as described in U.S. Published Patent Application No. 20090219019 (assigned to Schlumberger Technology Corporation), the entire disclosure of which is hereby incorporated by reference. An example micro-NMR device can be micro fabricated on a millimeter or sub-millimeter scale, and consist of a sample tube surrounded by an antenna that works as transmitter and as receiver. Moreover, micro-NMR devices can be made very small and a correspondingly low power utilization. However, the teaching of the present disclosure is not limited to the particular micro-NMR devices disclosed herein; rather, any suitable micro-NMR devices that can be fit in the borehole can serve this purpose.
Various needs in the art exist for systems that incorporate micro-NMR devices to analyze formations, samples, and/or the fluids therein. The present disclosure describes various systems and methods for utilizing micro-NMR devices in various applications.